Slot antenna with horn



1961 v. H. RUMSEY ETAL 2,996,715

SLOT ANTENNA WITH HORN Filed March 10, 1955 3 Sheets-Sheet 1 INVENTORS. V/C'TOE l7. Ell/756' Y 6775470 If WHLTEE A J W mo Aug. 15, 1961 v. H. RUMSEY ET AL 2,996,715

SLOT ANTENNA WITH HORN Filed March 10, 1955 3 Sheets-Sheet 2 IN VEN TORI).

v/croe l7. zuMsE 5' mm 70/) h. 14/0475 2 7 BY 4 J 5 Sheets-Sheet 3 INVENTORS.

Aug. 15, 1961 v. H. RUMSEY ET AL SLOT ANTENNA WITH HORN Filed. March 10, 1955 e mm 0 W m U E EHJ H Hw Mm 574 w m .Wa my V CW I. all.

United States Patent 2,996,715 SLOT ANTENNA WITH HORN Victor H. Rumsey, Urbana, Ill., and Carlton H. Walter, Columbus, Ohio, assignors to the United States of gmerlca as represented by the Secretary of the Air orce Filed Mar. 10, 1955, Ser. No. 493,580 3 Claims. (Cl. 343-767) This invention relates to a slorn antenna and particularly to a traveling wave slot with a horn aperture constituting a shaped :beam antenna especially suited to operation in the microwave region.

In the application of antennas to vehicles, particularly high speed vehicles, such as aircraft, it is desirable to provide an antenna which creates a minimum disturbance of the outer surface of the vehicle. Also, it is usually desirable that the antenna be highly directional and have low side lobe losses.

It has heretofore been proposed to provide a slot antenna which consists essentially of a traveling wave slot in the side of a rectangular waveguide. These slot antennas have been undesirable because of the wide conical -beam and high side lobes.

' The present invention improves the characteristics of the slot antenna by providing a horn operative tomatch the slot to free space, the diverging angle of the horn being effective to reduce the width of the conical beam to improve the focusing action and to materially improve the poor radiation characteristic of the normal slot/ 1f has been found that the geometry of the horn may be modified along its length to control the radiation amplitude along the slot with very little change in phase velocity.

It is accordingly an'object of the invention to provide an improved antenna.

It is a further object to provide a slorn antenna.

It is another object of the invention to provide an antenna giving a highly directional beam.

v Other objects and advantages of the invention will be apparent from the following specification taken in conjunction with the accompanying drawings in which:

FIGURE 1 is a perspective view of a slot antenna mounted flush in a ground plane;

FIGURE 2 is a view similar to FIGURE 1 and showing a tapered slot antenna;

FIGURE 3 is a perspective view of a slorn antenna;

FIGURE 4 is a perspective view of a tapered slorn antenna;

FIGURE 5 is an elevational view of a tapered slorn mounted in a ground plate;

FIGURE 6 is a top plan view of the mounted slorn antenna;

FIGURE 7 is a graphic presentation of the conical pattern of the slorn antenna; and

FIGURE 8 is a similar presentation of the E-plane characteristic.

In the construction according to FIGURE 1, a slot type antenna is constructed by providing a waveguide 10 with an elongated slot 12 in one of the faces, preferably a narrow face. The slot 12 is usually mounted flush with a ground plate 22 such as the outer skin of a vehicle. If such an antenna is oriented for maximum signal and then rotated about its axis, the resulting pattern 26 (see FIG. 7), known as a conical pattern, illustrates the width of this beam. The half-power points are found to subtend an angle of approximately 150. It was found that the typical wide conical angle of this type antenna could be improved by changing from the usual transverse electric feed to a transverse magnetic feed, as shown, by coupling the coaxial feed cable 14 into the end of the waveguide tive.

\ slot.

2,996,715 Patented Aug. 15, 1961 ice 10 with the inner conductor 16 extending substantially axially within the waveguide 10.

In the modification, according to FIGURE 2, a rectangular waveguide 18 is provided with a tapered slot 20 opening into a ground plate 22 with the waveguide 18 being provided with a transverse electric feed by having a coaxial cable 24 with the central conductor 26 ex tending transversely of the narrow width of the waveguide 18 adjacent to one end of the slot 20. In orderto maintain a constant amplitude distribution along the: length of the slot 20, the slot is reduced adjacent the.- feed and flared outwardly away from the feed point so that the amplitude of energy leaving the slot is substantially constant from end to end of the slot. This follows. the well-known fact that as energy leaves the slot 20 less. energy remains to radiate from the remainder of the slot. Therefore, the tapering of the slot 20 compensates for the reduced energy in the waveguide so that the radiation is substantially constant throughout the length of the slot.

It has been found that providing the slot with the transverse magnetic feed and with the tapered slot materially increases the focusing ability and consequently improves the operation of the antenna.

The beam width in the vertical pattern in the plane through the long axis of the slot and parallel to the ground plane is determined primarily by the length of the slot 20 and may be found from various known formula. In practice, it has been found that slots 20 having a length of the order of 10 to 15 wavelengths have been quite effec- In the construction, according to FIGURE 3, diverging horn members 30 and 32 are provided to match the narrow slot 34 to free space and operative to greatly improve the normally poor radiation characteristic of the The resulting slorn antenna has a good focusing action to produce a narrow conical pattern and further permits the antenna to be oriented for desired direction and also be mounted flush with a surface 22 as shown in FIGURES 5 and 6. It has been found that horn members 30 and 32 having a radial length R of approximately five wavelengths provide good focusing action without being undesirably cumbersome. A slorn having a length l of 10 to 15 wavelengths, a radial distance R of five wavelengths and a flare angle 1/ of 60 produces a conical pattern with a beam width of approximately 30 at the half-power points, as shown at 54 of FIG. 7.

The geometry of the slorn lends itself very well to the suppression of the side lobes '56 of FIG. 8. The amount of radiation along the slot is controlled by varying the geometry of the horn along the length of the antenna. An amplitude distribution that gives negligible side lobes can be obtained by providing a taper 36 and 38 on the horn members 30 and 32. Experiments indicate that the taper is not particularly critical. Tests show a remarkable suppression of the side lobes with very little change in the beam width of the conical pattern.

It is difficult to formulate the exact relation between amplitude distribution and horn taper. However, qualitative considerations have led to good results. By considering the antenna as a continuous array of sectional horn elements the length and flare angle ,0 of each sectional element can be selected to control the energy flow into the slot 34 in a prescribed manner. Usually it will be found that the flare angle 1,0 can be kept constant and only the length l of the slot 34 varied. The angle of maximum radiation is determined by the phase velocity along the slot. This is controlled by the geometry of the slot, the dielectric with which it is filled, and the type of excitation. The beam width in the vertical pattern is determined by the length of the slot. In this case the slot 34 is approximately 13 wavelengths along and' produces a half power beam width of about; 10 degrees. The '60 degree flare angle 1/ and the average horn length R of approximately five wavelengths produces the 30. degrees transverse beam. Remarkably low side lobes. are obtained by, the taper which results in amplitude. distribution that is approximately sinusoidal. The straight line taper of FIGURE 4 was used because of itssimplicity. It was found, that Xcould be carried from, to L/2 with good side lobe, reduction. In the interest of minimum transverse beam width X should be as long as is practical. for a given side lobe level.

When it is desired to mount, the slorn antenna flush with a surface other considerations are involved. A sketch of the flush mounted slorn is, shown in FIGURES 5 and 6. End plates 40 and 42 are provided at each end, of, the horn members 30 and 32. It. has been found that the end plates 40 and 42 must be substantially parallel to the aXis of maximum radiation in order. to suppress. reflections at the plates; The elfective radial length R then becomes the average of the length. R, and R of the end plates 40and' 42. The length l of the slot 47 in the waveguide 44 determines the radiation angle 0m. The waveguide 44 ismounted at the angle to the ground plate 22 to determine the beam direction angle 6. The length l of the slot 46in the waveguide 44 is related tothe length L of the aperture 48 in, the ground plate as indicated by the formula Inorder; to prevent a. reverse; feed lobe, it. is desirable. to

absorb or dissipate the energy reaching the end of the waveguide 44. and adissipativeload 50 is placed in the end of the waveguide 44.

The transverse beam. Width is still determined by the excitation; the flare angle 0,. and R whereR is the average length of the end, plates140 and 42. The beam.

width inthe vertical pattern is. determined by the length L ,of the aperture 48 in the, ground plane 42. Broadside radiation is obtained by slanting the feed end of the antenna in FIG. 5 away from the ground plane to permit a to be, 90, since in practice 0m is limited to about 80".

In a practical construction of the slorn type antenna itis customary to utilize a flush mounting with an angular disposition of the rectangular waveguide 44 and to. utilize transverse magnetic feed so that a narrow conical pattern will be secured as indicated at 54 in FIGURE 7 and the end plates together with the tapered horn construction reduce the substantially minimum side lobe patterns 56 as shown in the E-plane characteristics of FIG- U.RE 8.

Inthe operation; oftheslorn type, antenna the length l will be selected to secure the desired vertical pattern and the horn angl'e n will be selected to provide the radiation pattern and to suppress the side lobes of the E-plane pattern. In general, the traveling wave slot 46 will be mounted at an angle to therground plane 22 so that radiation may be. directed substantially longitudinally of the supporting vehicle:

For. purposeeof exemplification, particular embodiments of the invention have been shown and described according to the best present understanding thereof. However, it will be apparentto those:=skilled' in the. art that various changes and modifications in the construction and arrangementof. the parts thereof. may be readily resorted to without,departingfromthe; true. spirit of the invention.

We. claim;

1. A directional antenna comprising a rectangular waveguide, one. side, of. said waveguide. being open to provide an, elongatedslot, means operatively connected to provide. excitationinsaid' waveguide, an outwardflaring horn mounted on said waveguide and communicating with said slot. throughout. its length, said horn having a radial length of the order of five wavelengthsv and a flare angle of. the, order of.6fl. degrees.

2. A directional antenna comprising, a rectangular waveguide, one side;of, saidwaveguide. being open toprovideqan. elongated slot, means operatively connectedto provide excitation in said waveguide, an outwardly flaring horn. mounted. on. said. waveguide. and communicating withsaid slot, saidhornxbeing tapered, end. plates on. said tapered. horn, said end. plates being essentially parallel. to rays. of maximum radiation from. the slotted. wave. guide. a ground.plane-having an opening adaptedv tore.- ceive. saidhorn; said horn open. flush. with. the, outer surface. of the. ground plane.

3. A directional. antenna. system comprising aground plane, a rectangular.- waveguide mounted in angularly spaced relation to said ground plane, the side of said waveguide presented to. said. ground plane being open to provide an elongated slot, means operatively. connected to provide excitation inv said waveguide, an outwardly flaring horn mounted on said waveguide. and. communieating with saidslot throughout the length thereof, end plates on. said. horn, said endplate. being substantially parallel to rays of maximum radiation from theslot in said waveguide, said ground plane. having an opening adapted toreceive said horn, said. horn terminating flush with the outer surface of saidground planewhereby said horn is t tapered longitudinally.

References Cited in the file of this patent UNITED STATES PATENTS 2,405,242 Southworth Aug. 6, 1946' 2,433,368" Johnson eta-1 Dec. 30, 1947 2,594,409 Feldman Apr. 29-, 1952 

